In the past, asbestos has been the principal material used for heat insulation and sealing where the material is subjected to elevated temperatures. Asbestos fibers are plentiful and are readily combined into laps, roving, and yarn for various end uses by conventional textile apparatus. Asbestos materials tend to flake or slough off fibers unless specially treated, but such tendency has not been a significant deterrent to the use of asbestos because of its abundant supply and relative ease of handling in manufacturing.
In recent years, fiberglass has been developed as a substitute for asbestos where the coarseness of the asbestos material is unsuitable for the end use and where the fine filamentary characteristics of fiberglass provide properties which justify the normally higher cost of substituting fiberglass for asbestos.
A significant advantage of fiberglass is that it has considerable tensile strength in comparison to asbestos and in continuous-filament fiberglass products, the fiberglass tensile strength is relatively independent of the degree of twist in the fiberglass strands so that fiberglass may be fabricated with a looser or less compact fibrous structure.